1. Field of the Invention
The present invention generally relates to the treatment of industrial process water to control the growth and deposit of microorganisms, and more particularly to the treatment of industrial process water with a chlorine containing oxidant and a bromide salt in such a way that copper corrosion is reduced to a certain minimal level.
2. Description of the Related Art
Many industrial process waters are contaminated with microorganisms, particularly bacteria. Industrial process waters are used in the cooling towers and condensers of electrical power plants, and in many manufacturing plants such as paper mills.
Fouling caused by microorganisms in the cooling systems of power plants is an especially troublesome problem. The temperatures inside a typical condenser provide an ideal environment for the growth of microorganisms. Even a few thousandths of an inch of slime deposit on a condenser tube has been shown to affect condenser efficiency, plant heat rate and maintenance costs. The slime layer forms a sticky substance which allows silt and other particles to adhere to the surface of the condenser tube. The heat transfer loss due to this insulating layer has been estimated to cost the electric utility industry up to $400 million a year in additional fuel costs. Moreover, corrosion and pitting can occur under the slime and silt deposits, causing long term damage to the cooling system.
The least expensive method of controlling biofouling is to treat the process water by chlorination with either gaseous chlorine or liquid sodium hypochlorite. But chlorination practices have been strictly regulated by the Environmental Protection Agency ("EPA") due to toxic byproducts which are believed to form. This has required the electric utility industry to accept chlorine minimization and alternatives to chlorine to meet the strict chlorine discharge limits while maintaining adequate plant performance. The chlorine discharge limits are specified by a certain minimum value of the "total residual chlorine" (TRC). A typical minimum value of TRC is 0.2 mg/l.
There are a number of biocide alternatives to chlorination which are capable of reducing the TRC value to 0.2 mg/l. These alternatives include water treatment with sodium sulfite, sulfur dioxide, chlorine dioxide, and bromine. But in the amounts required for comparable biocide activity, sodium sulfite, sulfur dioxide, and chlorine dioxide are far more expensive than chlorine. Bromine is also more expensive than chlorine, but its biocide activity is greater. Consequently, bromine is one of the most cost-effective biocide alternatives to chlorination.
The increased biocide activity of bromine is not due to the elemental bromine itself, since it is well known that chlorine is a much stronger oxidant than bromine. For both chlorine and bromine, the biocide activity is primarily due to their respective reaction products with water, which are hypochlorous acid and hypobromous acid. Bromine and chlorine are also compatible biocides, and can be used simultaneously in various amounts.
Elemental bromine, which is a liquid, can be injected directly into the process water stream to provide biocide treatment. For use with chlorine, both the chlorine and the bromine in the form of fuming bromine chloride liquid can be injected simultaneously into the process water stream. But fuming bromine chloride is relatively dangerous to handle.
A safe and economical method of providing hypobromous and hypochlorous acid in process water is to chlorinate the process water in the usual fashion, and to simultaneously inject a bromide salt. The hypochlorous acid produced during chlorination reacts with the bromide ions in solution to form hypobromous acid and chloride ions. In order to achieve a good conversion rate with this reaction, the reactants should be relatively concentrated, and therefore the hypobromous acid (or hypobromite) is produced as a treating solution which is injected into the process water stream. Due to the fact that hypochlorous acid is consumed in the reaction, the chlorine residual to bromide ratio can be varied to obtain a system which contains anywhere from a total bromine residual to a total chlorine residual. This is particularly important in systems with high ammonia levels since bromamines degrade more rapidly, and consequently, are not as persistent in the environment.
The preferred method of biocide treatment of process water with a bromide salt and a chlorine containing oxidant is further described in Sharp U.S. Pat. No. 4,451,376, herein incorporated by reference. In accordance with a typical chlorination process, a solution is prepared including a chlorine containing oxidant chosen from the group consisting of an inorganic hypochlorite salt, hypochlorous acid, and chlorine. Also a solution is prepared containing a water-soluble inorganic bromide such as sodium bromide, and an anionic polymeric dispersant such as low molecular weight copolymers of acrylic acid or methyl or ethyl acetate. These two solutions are mixed to form a treating solution according to the conversion reaction described above. The treating solution is then injected into the process water stream.
To convert a chlorination system to the preferred bromine system, a metering and pumping system for low viscosity fluids is provided to inject the sodium bromide and dispersant solution into the chlorination system. The preferred point of injection is either before or after the chlorination injection so that the sodium bromide and the chlorine containing oxidant are mixed together in a relatively high concentration aqueous solution. A brand of sodium bromide and dispersant mixture is sold commercially under the trademark "Acti-Brom" by Nalco Chemical Company, One Nalco Center, Naperville, Ill., 60566-1024. As a starting dosage, it is recommended that the chlorine dosage be reduced by 50 to 75%. Then for every ppm of Cl.sub.2 being fed at this reduced rate, 0.85 ppm of sodium bromide should be fed. In addition, the chlorination time should be reduced by 50%; since the hypobromous acid is a very active biocide, even further reductions in chlorination may be possible.
Laboratory and field testing of activated bromide chlorine mixtures for biofouling control in power plant cooling systems is described in F. Kramer et al., "Chlorine Minimization With A Chlorine-Bromine-Biodispersant Mixture," presented at the American Power Conference, 46th Annual Meeting, Apr. 23-25, 1984, Chicago, Ill., herein incorporated by reference. Toledo Edison had investigated several methods of biofouling control and found that the most effective method was an activated bromide chloride mixture. One of the concerns was whether the activated bromide-chlorine mixture was more corrosive than chlorine alone. Laboratory studies, however, indicated that corrosion rates for the activated bromide mixtures were similar to chlorine alone, but overall corrosion rates should be less due to the shorter treatment time. Specifically, mild steel, admiralty, and 304 stainless steel cupons were exposed to a chlorine and activated bromine-chlorine residual of 2 times and 1000 times the normal expected value. The data were taken for a 24 hour period. For activated bromine-chlorine, the respective corrosion rates at 2 times normal residual were 4.7 mpy (mills-per-year), 0.2 mpy, and 0.1 mpy; for chlorine alone, the corresponding rates were 5.1 mpy, 0.2 mpy, and 0.0 mpy. It was concluded that if the treatment dosage remains below 2 times the normal value for only 1 hour per day, there should be no substantial corrosions of any of these metallurgies.
Typically the treatment time to prevent biofouling in power plant cooling systems is about two hours per day. The chemical feed rates, for example, are 2400 pounds per day of chlorine, and 170 pounds per day of sodium bromide.
In order to optimize a biocide water treatment program for a particular plant, Nalco Chemical Company has developed and used a mobile laboratory including monitoring equipment for measuring critical water cooling variables (such as pH, conductivity, fouling factors, and corrosion rates) and a computer programmed for data acquisition and correlation of changes in water chemistry or other variables to fouling and corrosion problems. The computer program includes a model of the operation of a cooling water system which considers the concentrating effect of the cooling tower and possible ranges of pH and temperature. The model predicts the solubility of up to 150 different minerals including calcium, chloride, copper fluoride, iron, magnesium, sodium, ammonia, nitrate, phosphate, sulfide, silicate, sulfate, and zinc.
Corrosion rates are important for monitoring the depreciation of the condensers. So that corrosion will not substantially reduce the expected lifetime of the cooling system, it is desirable to limit the corrosion rate to less than 0.1 mill per year.
In recent years it has become desirable to reduce corrosion due to a belief on the part of the EPA that corrosion products are toxic. For good heat transfer, mechanical strength and corrosion resistance, cooling systems are typically made of copper nickel alloy, such as 70/30 copper-nickel. In many locations, the EPA has set limits on the discharge of copper, which results from corrosion of the copper-nickel alloy. These limits vary from one locality to another, and sometimes they place more stringent limits on the corrosion rate than the limits dictated by the limits of residual chlorine and the economics of plant operation.